1. Field of the Invention
This invention relates to improvements in methods and circuits for operating voice coil actuator/motors (VCMs) of the type used in mass data storage devices, or the like, and more particularly to improvements in such methods and circuits that may be used to move the head mechanism of such VCM to a parked position from an operating position.
2. Relevant Background
A well-known hard disk drive assembly (HDA) is a typical mass data storage device of the type to which the invention pertains. Generally the HDA includes one or more rotating disks that carry a magnetic media to which data may be written, and from which previously written data may be read. The data is written to and read from the disk by one or more magnetic heads or transducers that are a part of a voice coil motor (VCM) assembly, which moves the heads to the desired locations at which data is to be written or read.
An exploded view of a portion of a typical HDA 5 is shown in FIG. 1. The HDA 5 includes a VCM apparatus 10 in conjunction with a plurality of rotating disks 12. The VCM assembly 10 includes one or more arms 14 that are pivoted about a bearing point 16 to carry and move the heads or data transducers 18 radially inwardly and outwardly within the stack of data disks 12.
The outboard end of the arm 14 carries a coil 20 that is selectively energized by currents from VCM positioning circuitry 22. The outwardly extending end 24 of the arm 14 is located between two horizontal magnets 26 and 28, which are mounted to base plates 30 and 32. The base plates 30 and 32 and magnets 26 and 28 are spaced apart by spacers (not shown) to allow the arm and coil portions 24 and 20 to freely swing between the magnets 26 and 28. The plates 30 and 32, spacers, and magnets 26 and 28 are securely fastened to the base plate 34. A top cover plate 35 encloses the top side of the base plate 32. The two plates 32 and 35 may physically touch or barely touch each other. Thus, as the currents from the VCM positioning circuitry 22 are applied to the coil 20, magnetic fields are established by the current induced field of coil 20 that can precisely position the heads 18 at a desired location under control of the VCM positioning circuitry 22.
When the apparatus 5 is powered down, typically the head mechanism is moved to a position (not shown) at which the heads 18 are “parked” or “landed”, often at the inner radius of the disk. In other cases, such as when the head is parked on a ramp, they may be parked along the outer radius of the disk. In order to properly move the heads to the park position, generally a driving current is applied to the coil 20 that is of sufficient magnitude to bring the head assembly just to the park position. However, it will be appreciated that if the head mechanism is overdriven, the delicate head mechanism and other parts of the disk assembly may sustain damage. On other hand, if the head is underdriven, the head mechanism may not reach the park position, which may result in loss of the air bearing between the head and disk surface, which may also cause damage both to the head mechanism and to the underlying magnetic media of the disk assembly 12 above which the heads 18 fly.
The heads are positioned by the positioning circuitry 22, also referred to herein as a servo circuit, of the type shown in FIG. 2, which also operates in the retraction or parking of the heads to their landing zone or landing ramp. The servo circuit 22 may incorporate a floating-terminal BEMF detection scheme (FLBD) 23 in its design to control the retract of the heads to their parked position. The purpose of FLBD is to extract the BEMF signal from the VCM terminal voltage difference, Vpn=Vp−Vn, at nodes 62 and 58. This is done normally by turning off all four FET's 44–47 to let Vp and Vn on nodes 62 and 58 float for a short time. After the flyback current in the VCM coil decays to a predetermined level, which is defined to be at or near zero and the rate of change of the current is also at or near zero, Vpn theoretically will approximate the BEMF voltage, since with no current, there should be no voltage drop across resistor R0 60, resistor RSEN 55, and the motor inductor L0 49.
One technique controlling a VCM is shown in U.S. patent application Ser. No. 09/388,508 now U.S. Pat. No. 6,184,645, filed Sep. 1, 1999, incorporated herein by reference. One technique measuring the BEMF of the coil of the actuator used in said application Ser. No. 09/388,508 now U.S. Pat. No. 6,204,629 is shown in U.S. patent application Ser. No. 09/193,803, filed Nov. 17, 1998, incorporated herein by reference.
With reference again to FIG. 2, the circuit 22 includes a VCM predriver circuit 42 that provides signals to drive transistors 44–47 in a selective manner by which current flows through the coil 20 of the VCM in one direction or the other to move the head of the VCM in the desired direction. Thus, for example, to move the head in one direction, transistors 44 and 45 are turned on to establish a current flow path between the voltage terminal 51 and a ground terminal 53 to move the head in a first direction. To move the head in the opposite direction, transistors 46 and 47 are turned on to establish a current flow path through the motor coil from the motor driving potential 51 to ground 53. In the circuit embodiment shown, a sense resistor, RSEN, 55 is shown in series with the motor inductance, L0, 49 and the node Vn 58. The resistance of the coil 20 is shown as resistor 60, in series between the motor inductor 49 and the node, denoted Vp. For clarity, the remainder of the circuit elements of the VCM model 50, described in detail below with reference to FIG. 3, are lumped into element 61, except for the capacitance C0 56 and the resistor Rh 66, which can be disregarded.
As mentioned, when the head is to be moved to the park position, one method that may be employed is to tristate the transistors 44–47, wait a period of time to allow the flyback current to occur and dissipate down to a predetermined magnitude. Thus, after the flyback current has dissipated to the predetermined level, the voltage appearing between nodes 62 and 58 is measured, which, at least in theory, should represent the BEMF developed across the coil 20. Since the BEMF has a value almost directly proportional to the speed of the coil of the VCM, knowing the velocity of the coil 20 enables the precise required drive current to be determined that will properly move the heads to the parked position at a controlled velocity.
However, in practice, it has been found that the BEMF that is measured using the prior art techniques does not always accurately represent the correct velocity of the coil 20, and, consequently, the head assembly controlled thereby. As discussed below, we have determined that this is due at least in part to the influence of eddy currents induced in the structures adjacent the coil 20 of the VCM on the voltage induced into the coil during its movement at the same time that the BEMF is measured.
What is needed, therefore, is a method and circuit for more accurately determining the BEMF when the VCM drivers are tristated to enable the current needed to be applied to the coil to properly park the heads at a controlled velocity to be determined.